The invention is based on a priority application EP03360007.3 which is hereby incorporated by reference.
The present invention relates to an optical transmission system comprising at least one transmitter, at least one transmission line, at least one optical fiber amplifier, and at least one receiver, the optical fiber amplifier being designed to show a flat characteristic of output power versus wavelength.
Such optical transmission systems are per se known.
In general, gain tilt is the measure of the slope of the wavelength dependent gain of a fiber amplifier (EP 782 225).
It is possible to install an optical transmission system such like a Wavelength Division Multiplexing(WDM) system only with erbium doped fiber amplifiers (EDFA) as amplifying components.
Modern EDFA are two stage amplifiers with mid-stage access for dispersion compensation (with Dispersion Compensating Fiber) or Optical Add and Drop Multiplexer (OADM). A mid-stage Variable Optical Attenuator is used to control the tilt of the EDFA. We can consider the following EDFA:    P_in=−5 dBm, P_out—1_stage=14 dBm,    P_in—2_stage=5 dBm, P_out—2_stage=20 dBm
A large number of EDFA are available on the market. Hence, the above values should rather be understood as exemplary than be understood as limiting the present invention's area of application.
Of course, a certain optical signal to noise ratio (OSNR) has to be achieved at the end of the transmission line.
State of the art systems utilising erbium doped fiber amplifiers are usually designed to have a flat overall output spectrum.
However, it is not easy to predict the performance of a terrestrial system with good accuracy before installation. Therefore, a system that has been designed to have a flat output may, after installation, not achieve the desired optical signal to noise ratio. Further, the characteristics of an installed system may deteriorate due to ageing or due to repair of the fiber during operation.
Accordingly, the transmission line may suffer from optical signal to noise ratio degradation. Even further, OSNR degradation may be caused after a first installation of the system by increasing the number of channels of the system.
It is, therefore, desirable to have a certain OSNR margin in the system that may compensate for the above mentioned degradation. An OSNR margin is understood as a margin, or distance between the target OSNR at the end of the transmission system and the OSNR needed in back to back to have an admissible BER.
In general, the Bit Error Rate (BER) of the system has to be better than 10−13. In order to achieve this performance, Forward Error Correction (FEC) is used. With a BER of 10−13 before correction, a BER that is better than 10−13 is achieved after FEC. However, detrimental effects may occur that can degrade the BER. Examples of such effects are non linear effects that are caused by to much power being fed into the fiber, chromatic dispersion of the fiber, Polarization Mode Dispersion (PMD), and, as mentioned above, ageing of the line as well as losses due to repair on the transmission line.
The impact of these effects is measured as an OSNR penalty. For example, we can consider that an OSNR improvement of 1 dB is needed to compensate for dispersion problems, 1 dB for PMD-compensation, 1 dB for compensating non linear effects, 2 dB for ageing and repair, and so on. In back to back, an OSNR of 12 dB/0.1 nm is needed to achieve a BER of 10−3. To guarantee, after transmission, a BER of 10−3, an OSNR of 12+5 dB=17 dB/0.1 nm may be needed. In this particular case, the margin would be 5 dB.
If this OSNR margin is not achieved at the end of the transmission system, the OSNR can be improved by improving the Noise Figure (NF) of some optical amplifiers. The Noise figure of Raman amplifier (typically NF=0 dB) is better than that of EDFA (typically NF=5 to 6 dB).
The NF of a Raman amplifier with NF1 and G1 and a subsequent EDFA with NF2 is NF1+NF2/G1 (NF1, NF2 and G1 are expressed in linear).
However, Raman amplifiers are expensive and should therefore only be installed when necessary.
Accordingly, it is desirable to have the opportunity to install a Raman amplifier after a first installation of the system. Such a supplementary installation should be possible in connection with a first installation of the system, when the system does not achieve the desired performance. It should, further, be possible to add Raman amplification after years of operation. However, such a supplementary installation has, prima facie, certain drawbacks.
First of all, the desired characteristics of an optical transmission system such as a WDM system should be conserved when further amplification is added. As already mentioned, the desired characteristics include a flat system output. If a system is designed to include a Raman amplifier, a flat system output is usually achieved by combining a Raman amplifier with flat gain characteristics with an EDFA, the Raman amplifier and the EDFA being designed to have a flat output spectrum when combined with each other. However, EDFA with high input powers (due to Raman preamplification) require a specific EDFA design. As a result, a particular EDFA is designed to provide one particular flat output power when fed with one particular flat input power.
If the input power is changed, for instance by incorporating an additional Raman amplifier, the output power of the system will be tilted, even if the additional Raman amplifier provides a flat input to the EDFA. A tilted output means for example, that channels allocated to longer wavelengths will have more power than channels allocated to shorter wavelengths.
It is known in the art, e.g. from U.S. Pat. No. 6,275,329, to compensate for a tilted output by utilizing Variable Optical Attenuators (VOA). However, a VOA allows to keep the flatness of the EDFA output but at the expense of the OSNR.
Further, the incorporation of an additional Raman amplifier into an existing system would, prima facie, require to cut the system in order to allow for installing the additional Raman amplifier. In other words: The whole system would need to be switched off, the additional Raman amplifier, had to be put in and, the whole transmission line had to be switched on again.